I was taught to "make a little tent" of loose skin and inject into the base of it. I use a short, fine gauge needle, clean one for each sheep, and rub the injection site to disperse the adjuvant. I inject in the neck and it's where the sheep have least wool at this time of year. If I have to give two injections in fairly short succession (Ovivac P Plus, for example) I inject first in the left side and then in the right.